Conventionally, formed articles made of heat resistant polymers have been used in an electrical insulation field where heat resistance is required. Especially, formed articles using aromatic polyamides (hereinafter, referred to as aramids) are excellent industrial materials having heat resistance, chemical resistance, and flame retardancy attributable to the molecular structures of the aramids. In particular, a paper (product name: NOMEX (registered trademark)) made of a fibrid and a fiber of poly(meta-phenylene isophthalamide) (hereinafter referred to as meta-aramid) has been widely used as an electrically insulating paper excellent in heat resistance.
In general, almost all formed articles using meta-aramid are colored in white, and the above-described meta-aramid paper is also white or transparent white. Meanwhile, the meta-aramid paper is used in electrical insulation applications, for example, in such a manner that the meta-aramid paper is shaped into tape with a narrow width, and wound around an electric conductor, or that the meta-aramid paper is cut into a predetermined shape, and if necessary, shaped, and then inserted or fitted along the shape of an electric conductor, a casing, or the like. However, the meta-aramid paper has the following problems and the like. Specifically, for example, when multiple lead wires around which the meta-aramid paper is wound are bundled together and used as an electric cable, the appearances of all the lead wires are the same, and it is hence difficult to determine which lead wire is to be connected. In addition, when the meta-aramid paper is used after being attached to or fitted into a casing of a similar color or the like, it is difficult to find out whether an insulating paper is integrated. As a solution to these problems, there has been a demand for a colored aramid paper.
Processes for producing a colored aramid paper can be roughly classified into processes in which a meta-aramid fiber and a meta-aramid fibrid serving as raw materials are colored in advance, and a sheet is formed therefrom, and processes in which an aramid paper is colored after its production.
Many methods have been proposed for coloring a meta-aramid fiber. Examples of disclosed methods include a method in which a dope is dyed with a pigment (for example, UK Patent No. GB1438067), a method in which a meta-aramid fiber is dyed by using a cationic dye (for example, Japanese Patent Application Publication No. Hei 09-95870), a method in which a functional group is introduced into a meta-aramid fiber to improve the dyeing properties, and then the meta-aramid fiber is dyed by using a cationic dye (for example, Japanese Examined Patent Application Publications No. Sho 44-11168), and the like. It is conceivable that a meta-aramid fibrid can be colored by using a similar method. However, the dope-dyeing has problems such as the limitations on the range of hue, and increase in production costs because of the use of the pigment in raw material production, which requires cleaning of a line and the like. In addition, the meta-aramid fiber has lower dyeing properties than fibers which are used for clothing and to which the dyeing process is applied in general, such as, for example, polyester fibers, acrylic fibers, and rayon fibers. Hence, the range of hue is limited also for this reason. In addition, there is a concern over such problems that since the fiber and the fibrid are different in dyeing properties, non-uniformity in color tends to be caused when a sheet is formed from them, and that because of exposure to high-temperature wet heat, a sheet formed from them has decreased mechanical strength.
In addition, a method may be employed in which an aramid paper is dyed after its production. However, this method has such problems that the difference in dyeing properties between the fiber and the fibrid tends to cause non-uniformity in color as described above, and the wet heat treatment tends to cause wrinkles, and lowers the mechanical strength.
Note that generally used aramid papers are calendered at high temperature, and few functional groups are present on the surface of the aramid itself. For these reasons and the like, aramid papers have such a problem that the aramid papers are more resistant to penetration of a coloring liquid, and hence are more difficult to color uniformly than pulp fiber papers.